1. Field of the Invention
The present invention relates to an impregnated cathode of a cathode-ray cube (hereinafter, referred to as CRT), and more particularly to a low power impregnated cathode of the CRT.
2. Discussion of the Related Art
In general, in a wide CRT requiring high definition and high luminance, an impregnated cathode capable of emitting high current beam for a long time has been used.
Referring to FIG. 1, there will be explained the structure of the impregnated cathode according to the prior art.
A pellet 1 is installed in a cup 2, the pellet 1 being used for emitting electron. The cup 2 is coupled with an inner sleeve 4 by laser welding. The inner sleeve 4 is coupled with an outer sleeve 5 by a tab 3. A cathode support (not shown) is positioned near to the outer sleeve 5. Further, a heater 6 is mounted in the internal part of the inner sleeve 4, i.e., in the bottom part of the pellet 1.
In the process of the manufacture of the pellet 1, metal powder is made of tungsten powder or made by mixing the tungsten powder which rare earth metal such as Os, Ir, and Ru. The metal powder becomes a porous metal having the porosity of 20.about.30% by sintering. Then, the pellet 1 is made by impregnating the porous metal with electron emission material. The electron emission material consists of BaO, CaO, and Al.sub.2 O.sub.3 at the mole ratio of 4:1:1, 5:3:2, or 3:1:1. Further, materials such as Ir, Os, Ru, Os-Ru, W-Ir, and W-Os are sputtered at Ar gas atmosphere and then the surface 7 of the pellet 1 is coated with the sputtering material of the thickness of 150 .mu.m, so that the pellet 1 is operated at the cathode temperature of 950-1000.degree. C., (brightness temperature).
Meanwhile, there are two kinds of the impregnated cathodes, i.e., a normal impregnated cathode having heater current of 680 mA, and a low power impregnated cathode having heater current of 320 mA. In the normal impregnated cathode, the pellet 1 is designed to have the thickness H of 0.5 mm and the diameter D of 1.5 mm.
In order to implement the low power impregnated cathode, the size of all the cathode components like the pellet 1 should be reduced. Accordingly, according to the prior art, the diameter D of the pellet 1 is reduced to 1.0 mm and its thickness H thereof is maintained at 0.5 mm.
However, there are provided the following problems in the low power impregnated electrode according to the prior art.
First, the amount of electron emission material to be impregnated in the pellet is reduced, since the size of the pellet is reduced in order to lower the heater current. Accordingly, the heater current is reduced and also the electron emission capability is reduced.
Second, since the cathode consists of the pellet, cup, and inner sleeve, and the cap having the pellet is connected with the inner sleeve by welding, a lot of components are used. Further, the pellet might be damaged because of the welding of the cup and the inner sleeve.
Third, the loss due to heat conduction can be increased because the contact surface between the outer sleeve and the cathode support is wide.
Lastly, the diameter of the outer sleeve of the low power impregnated cathode is smaller than that of the outer sleeve of the normal impregnated cathode, so that the components like the cathode support of the normal impregnated cathode should be changed.